I just accepted a visiting chemist that petitioned to study at my fort. Before unpausing, I looked at his thought page.
He 'feels vengeful after joining an existing conflict'. Zooming to his location, I see he is standing next to the elected mayor of my fort, who also 'feels vengeful after joining an existing conflict'.
All dwarves also have 'I have a part in this! I will take revenge!' as current thought, top line of the thought page.
Inquiring further, I find that over half of my fort's population has this in their thoughts screen. There is nothing in the combat reports, except my broker killing a wild dingo with a wooden cup, actually managing to get it stuck in the dingo's skull, and my marksdwarves who are sparring outside shooting at passing keas (wonderful how well this works now. Give your marksdwarves both a barracks as well as a shooting range. They'll train at the barracks when there's no training bolts, and as a bonus, they shoot at anything that gets too close.
There's also nothing in the justice screen. Besides, all my dwarves are way too happy to fight. I've only ever seen one fistfight in the very first year of my fort. The instigator was a 'very susceptible to disease' case, who died of alcohol poisoning not much later.
There's also no loyalty cascade going on. No fighting at all.
After unpausing for 10 seconds, checking back, the thoughts are gone from every dwarf. So what did those thoughts mean? Did witnessing my marksdwarves fight a kea, or my broker fight a dingo give them that thought temporarily?